Question on the curve that astro stacking creates

[Don Samuels]

I am trying to understand the adjustment curve layer that the built in astrophoto stack feature generates. When the stacking is done, there are two adjustment layers. First is a levels adjustment, and then a curves adjustment. When I click in the little white box on the curves adjustment, I get this:

The histogram is barely visible at the bottom. Then when I click on the box next to where it says master:

and choose one of the selections (by the way what is that box called?)

This now lost all of the adjustments and you can not get them back. Behavior is the same on Windows 10 and Mac OS. Windows 10 is at 1.10.5.1342

A few questions

1) if I want to take the curve as Affinity thinks is best, why is the histogram on the curve box so low as to not be useful

2) the box where I clicked grey, how come that kills the adjustments that Affinity applied? 

3) what is that box called? When I hovered on it I did not get a pop up?

4) is there a technical documentation that explains each of the windows like adjustment curves?

Meanwhile, what I was doing is capturing M51 (Whirlpool galaxy) with a 200 mm lens and a equatorial mount. Did not spend much time after stack but quickly got something that tells me it will be usable as I learn more:

Thanks

Don

 

[NotMyFault]

29 minutes ago, Don Samuels said:

A few questions

1) if I want to take the curve as Affinity thinks is best, why is the histogram on the curve box so low as to not be useful

2) the box where I clicked grey, how come that kills the adjustments that Affinity applied? 

3) what is that box called? When I hovered on it I did not get a pop up?

4) is there a technical documentation that explains each of the windows like adjustment curves?

Hi,

1) The histogram is shown in a technical correct way, but highly useless, especially in RGB/32 mode with linear tone curve.

it counts for 256 brightness values how many pixel of that brightness exist. If you have a smooth gradient from black to white, the full height is used. If you have a unequal distribution of brightness, like in astro, most pixels are pure black or of a dark color. This one spike leads to 99% of pixels getting added to one or very few columns in histogram, giving a 1px wide spike at the left edge (often invisible as side bug), and downscaling the relevant lightness values  what is missing is a practical scaling, excluding the useless spike of blacks, and giving the relevant colors full scale on y axis 

2) it changes the color mode, and all other inputs depend on color mode, e.g. number of color channels plus alpha 2, 4, 5, so must be reset

3) color mode

4) Online help explains it in depth, or watch the tutorial videos  

https://affinity.help/photo/English.lproj/pages/Adjustments/adjustment_curves.html